1. Field of the Invention
The present invention relates generally to fuel control systems for internal combustion engines and, more particularly, to a cold start fuel control system.
2. Description of the Prior Art
Most modern day internal combustion engines of the type used in automotive vehicles include a plurality of internal combustion chambers. An intake manifold has one end open to ambient air and its other end open to the internal combustion chambers via the engine intake valves. During a warm engine condition, a multipoint fuel injector is associated with each of the internal combustion engines and provides fuel to the internal combustion engines. The activation of each multipoint fuel injector is typically controlled by an electronic control unit (ECU).
During a cold start engine condition, however, a single cold start fuel injector is often times provided to supply fuel to the air intake manifold to the engine. The single cold start fuel injector injects sufficient fuel into the air/fuel intake passageway to provide fuel for all of the cylinders of the engine during engine warmup. As the engine warms up, the cold start fuel injector is gradually deactivated while, simultaneously, the multipoint fuel injectors are gradually activated in order to provide a smooth transition between the cold start fuel injector and the multipoint fuel injectors.
In order to ensure engine start-up during a cold engine condition, it has also been the previous practice for the cold start fuel injector to inject sufficient fuel into the engine in order to achieve a rich air/fuel mixture having a ratio in the range of 10:1 to 14:1. Even though such a rich air/fuel ratio is sufficient to ensure proper starting of the engine during cold starting conditions, the overly rich air/fuel ratio produces a relatively high amount of undesirable engine emissions, such as hydrocarbon and nitrous oxide emissions.
Such an overly rich air/fuel mixture has previously been required to ensure that there is sufficient fuel vapor within the internal combustion engine to ensure engine starting. Such vaporization of fuel is more difficult to attain during a cold start condition than a warm engine condition since the fuel is not vaporized by contacting hot portions of the engine, such as the intake manifold and internal combustion chambers.
While the previously known cold start fuel control systems have been sufficient to ensure proper starting of the engine while meeting prior governmental regulations, such systems are inadequate to meet the proposed future governmental regulations relating to exhaust emissions from automotive vehicles. For example, a United States emission regulations for CO, HC/NMOG and NO.sub.2 for the year 1991 are 7.0, 0.39 and 0.40 grams/mile respectively. For the model year 1997, the corresponding levels must be reduced to 1.7, 0.040 and 0.20 grams/mile respectively.